1. Field of the Invention
The invention relates to a granulated material for 3D binder printing, to a process for producing it, to a 3D binder printing process and to an object produceable using the granulated material or the printing process.
2. Related Art of the Invention
3D binder printing processes are processes for producing three-dimensional objects from a granulated material, in which a layer of the granulated material is placed on a base and then wetted with a binder liquid in predetermined regions which in each case correspond to a layer of an object that is to be produced. These steps are repeated until a predetermined three-dimensional object has been built up completely from joined-together granulated material particles. Then, the excess of granulated material particles is removed and the object is revealed. In a first type of these processes, the granulated material particles are partially dissolved at the surface in the regions which have been wetted by the binder liquid, and the subsequent evaporation of the binder liquid causes the particles of granulated material to be bonded to one another directly in their edge regions by fusing together. A second type of these processes uses a binder liquid which contains auxiliary substances which remain behind in the wetted regions during drying, allowing the wetted particles of granulated material to be joined to one another by subsequent partial melting or sintering.
3D binder printing processes, in particular of the first type, are known from European Patents EP 0 644 809 B1, EP 0 686 067 B1 and the European Patent Application EP 1 099 534 A2.
Binder printing processes which join the particles of granulated material by partially dissolving them using the binder liquid have the drawback that the finished object is subject to significant shrinkage compared to the region of the layer of granulated material which was originally wetted with the binder liquid. The reason for this is that partially dissolved particles which are in contact with one another draw closer together under the influence of their surface tension, resulting in a more tightly packed arrangement than hitherto after the binder liquid has dried. This effect cannot readily be avoided, and indeed is even necessary to a certain extent in order to achieve sufficiently strong cohesion of the particles in the finished object. However, one serious drawback of this effect is that the shrinkage during the drying process leads to the formation of cracks in an object produced using a process of this type which exceeds a certain maximum size.
To combat this problem, binder printing processes have been developed in which the binder liquid contains additives which remain behind in the wetted regions of the layer after the liquid has dried and make it possible to join the particles in the wetted regions by the entire mass of powder being processed, including the regions which have not been wetted, being heated in such a way that the particles in the wetted regions sinter together under the influence of the sintering aid, but the particles which have remained unwetted do not sinter together.
One problem of this technique is that the sintering aids used are generally of a mineral type and are at best dispersible but not soluble in the binder liquid, which means that they cause considerable wear to the spray nozzles used to wet the granulated material.
A further problem of the known binder printing processes is that as a result of agglomeration of the granulated materials used, objects produced thereby tend to have a nonuniform, rough surface profile which does not precisely correspond to the profile of the wetted regions.